1. Field of the Invention
This invention relates generally to processes for the recovery of gold from gold-containing carbonaceous ores or mixtures of carbonaceous and oxide ores using chlorine leaching techniques. More specifically, this invention relates to the recovery of gold from carbonaceous ores or mixtures of carbonaceous ores and oxide ores in which a simultaneous chlorine leach and adsorption step is used, where the absorbent is a member selected from a group of ion exchange resins that selectively adsorb gold from gold-containing solutions.
2. Prior Art
In the context of this disclosure, oxide ores are gold-containing ores from which gold is easily extracted by conventional cyanidation techniques. "Carbonaceous gold-containing ores" or simply, "carbonaceous ores", denote refractory gold-containing ores which also contain organic carbonaceous matter. Organic carbonaceous matter substantially reduces the effectiveness of lixiviants during the extraction of gold from such ores by conventional cyanidation technology.
Carbonaceous gold-bearing ores are found widely in the United States and in other countries throughout the world. In some instances, carbonaceous ores can be found without the admix of oxide ores, however, the usual case is that carbonaceous ores and oxide ores are found in mixtures. The fraction of carbonaceous ore in the mixtures of carbonaceous and oxide ores can vary. These ore mixtures have the characteristic of not being amenable to standard cyanidation techniques because of their carbonaceous ore content. For example, less than about 50 percent gold extraction is obtainable from such ore mixtures when these ore mixtures are treated by conventional "straight" cyanidation methods. For this reason, it is usually more efficient to process the ore mixtures as if the ore mixtures were composed entirely of carbonaceous ore. Carbonaceous ores are not amenable to standard cyanidation techniques because carbonaceous and other deleterious impurities often associated with carbonaceous ores react with or "tie up" gold cyanide complexes. Typically, a carbonaceous gold bearing ore contains anywhere from 0.05 to 2.0 ounces of gold per ton of ore and has from about 1 to about 8 percent by weight total carbon. The organic carbon content of the ore is usually between about 0.05 and about 3 percent by weight.
Research into methods of recovering gold from carbonaceous gold-containing ores has led to the development of processes utilizing oxidation to render the carbonaceous ores more amenable to subsequent cyanidation steps. Oxidation in these processes is usually accomplished by a first step of oxygenation with air or another oxygen containing gas. A chlorination step when follows using gaseous chlorine to complete the oxidation step. In several of the oxidation processes, chlorination is the major oxidizing step, but oxygenation is used as a "preoxidation" step to reduce the amount of chlorine required in subsequent chlorination steps. Preoxidation can include oxygenation, chlorination, or combinations of these. The oxidation step is then followed by a cyanidation step in standard gold extraction methods.
One process using oxygenation and/or chlorination prior to a cyanidation step is delineated in U.S. Pat. No. 4,289,532 to Matson herein incorporated by reference. The disclosure of the Matson reference is illustrative of the complex processes presently required to recover gold from carbonaceous gold-containing ores. This process requires (1) the wet grinding of the carbonaceous ore, (2) a preoxidation step, (3) a chlorination step, (4) holding steps to eliminate hypochlorite ions, (5) a cyanidation step, and (6) an electrolytic operation to remove the gold from the solution. It is undesirable in certain geographical locations to conduct cyanidation operations because of potential environmental and safety hazards. For this reason and because of the expense of cyanide compounds, the gold extracting industry attempts to reduce or eliminate cyanidation steps in gold extraction methods.
One method for eliminating or reducing the degree of the cyanidation step in a gold extraction method is the extraction of the recoverable gold with only a chlorination step. In this method chlorine is injected into an aqueous ore slurry and converts gold into a soluble gold complex ion. The exact chemical structure of the gold complex ion is not well defined. Such extraction methods are known as chlorine "water leaching" methods and were popular during the 1800's. These methods were discontinuted after the discovery of more efficient cyanide leaching processes.
Several processes exist that were developed to improve the yield and/or the rate of recovery from the reaction of chlorine and gold in a gold recovery method.
U.S. Pat. No. 3,834,896 to Eisele et al. discloses a process whereby gold is recovered from ores by treating the ores at elevated temperatures with gaseous chlorine in the presence of a promoter comprising a compound containing iron, aluminum, or gallium. Activated carbon is then used to adsorb the resulting complex AuMeCl.sub.6 where Me is iron, aluminum, or gallium. The fuel expenditures to achieve the elevated temperatures of 200.degree. to 500.degree. C. required by this process make this process economically undesirable.
U.S. Pat. No. 4,131,454 to Piret et al. describes a process whereby gold and silver can be recovered from chloride solutions containing other heavy metals. The chlorinated solutions are subjected to a three step treatment. In the first step, finely divided activated carbon is added for reduction of gold to gold metal and adsorption of the gold metal by the carbon. In the second step, a reducing metal such as zinc dust is added to the solution. In the third step, iodine is used to selectively precipitate the precious metals. Melting is then used to recover a precious metal regulus. This process, because it requires heavy metals to be the main constituent of the recovery solution, has a narrow application and is not generally economical for the recovery of gold from carbonaceous ores.
U.S. Pat. No. 4,056,261 to Darrah discloses a system for recovering gold or silver from "mine run" dumps or crushed ores. The system uses an inclined surface for supporting a heap of the mine run dumps or crushed ores. An alkaline cyanide aqueous solution is distributed onto the heap. A "pregnant" or gold-containing solution results and is collected as a run-off from the inclined surface. Activated carbon loaded ion exchange vessels are used in series to strip the gold or silver from the run-off solution. This process is concerned with a method combining a resin with a cyanide solution and is, therefore, unable to reduce or eliminate the use of cyanide in a gold or other precious metal extraction process.
U.S. Pat. Nos. 4,128,462 and 4,139,432, each to Ghiringhelli et al. disclose processes for recovering precious metals from ores. These processes use cyanide complexing agents in conjunction with electroconductive cationic resins. These disclosures are primarily concerned with increasing the rate of reaction for the electrochemical deposition of precious metals from an ore per unit of time.
U.S. Pat. No. 3,985,552 to Edwards discloses a method for separating gold from palladium in solutions containing chloride complexes of these metals together with other platinum group metals. Base metals can be present in the solution. The method requires contacting of the solution for a sufficient amount of time to result in the extracting of substantially all of the gold or palladium values present within the ore with a water insoluble, non-volatile solvent extractant in an organic phase and having functional groups of the formula R.sup.1 --S--R wherein R.sup.1 and R may be the same or different group and have a carbon atom joined directly to the sulfur atom. This compound is capable of acting as an ion exchanger or complexing group and separates the two phases to recover the extracted gold and palladium. This patent is only concerned with the use of thioethers having the disclosed structure and the separation of the various disclosed precious metals in a chloride complex solution.
U.S. Pat. No. 3,656,939 to Boehm et al. discloses a process for the removal of palladium cations in a cation exchange resin bed from a chloride solution. A reducing agent is added to the chloride solution to precipitate the palladium. This enables the palladium to be selectively obtained at the exclusion of all other metals. The ion exchange resin identified in this patent is narrowly defined and the invention of this patent is not directed to the extraction of precious metals from refractory ores.
The precious metal extracting industry lacks an efficient chlorine leach method to extract gold from carbonaceous ores.